1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device in which a field-effect transistor (FET) having a silicon oxide interlayer film is formed on a compound semiconductor substrate, and more particularly to a method of evaluating the interlayer film in order to control the degradation of FET power characteristics due to electron trapping.
2. Description of the Related Art
When a high-output compound semiconductor FET is driven for an extended period of time, its power output degrades due to the effect of electrons trapped in the interlayer film, or between the compound semiconductor substrate and the interlayer film. A description of this effect is given in “Relationship between gate lag, power drift, and power slump of pseudomorphic high electron mobility transistors,” Solid-State Electronics 43 (1999), pp. 1325-1331 (hereinafter, Reference 1).
One method of assessing the degradation of the interlayer film employs Fourier-transform infrared (FT-IR) spectroscopy, as described in Japanese Unexamined Patent Application Publication No. 7-221150 (hereinafter, Reference 2). In this method, the change in the FT-IR spectrum of an interlayer oxide film before and after operation of the FET is measured, and the degradation of the interlayer film is determined from the change.
Reference 1 deals with mitigation of the degradation of power characteristics of a high-output compound semiconductor FET having a silicon nitride film, but does not address the degradation of power characteristics of a high-output FET having a silicon oxide film formed on a compound semiconductor substrate.
Since the method described in Reference 2 requires measured data to be obtained after operation of the FET, degradation evaluation takes time. This method is not suitable for use during mass production of semiconductor devices.
It would be desirable to have a more practical method of controlling the degradation of power characteristics of high-output compound semiconductor FETs having a silicon oxide interlayer film.